Field of the Invention
In recent years, research and development, as well as application to practical use, have been vigorously conducted in regard to bio-nursery for growing elite seeds and new spices by using bio-technologies such as tissue culture, cell fusion and gene recombination, as well as on plant factories intended for stably producing various pants with high product quality under an artificial environment which is free from natural conditions. A bio-nursery and a plant factory essentially require that light necessary for growing plants supplied from an artificial light source. Hitherto, lamps such as fluorescent lamps and BID lamps, e.g., metal halide lamps, high-pressure sodium lamps, have been used independently or in combination as the artificial light source. Among these lamps, fluorescent lamps are easy to handle and capable of being placed in close proximity to the object plants to satisfactorily illuminate the lamp. In addition, although the fluorescent lamps have been considered as being unsuitable as light sources for plant growth due to small light output per unit lamp as compared with BID lamps, compact fluorescent lamps having high output power have been developed. For these reasons, fluorescent lamps are nowadays expected to be one of the prominent artificial light sources for growing plants.
A fluorescent lamp for plant growth has been known which has two components: namely, a red fluorescent component and blue fluorescent component. The light emitting spectrum of this type of fluorescent lamp resembles that of the chlorophyll bio-synthetic action spectrum of plants. Typically, magnesium fluorogermanate activated with manganese is used as the red fluorescent component, while calcium tungstate is used as the blue fluorescent component.
Morphology of plants is a very important factor, particularly when the plants of high quality and high commercial value are to be produced. The elongation growth which largely affects the morphology of a plant is closely related to ratio of photon fluxes contained in two wavelength bands which are centered at 660 nm and 730 nm, i.e., the ratio between the photon flux included in the wavelength band of 600 to 700 nm and that in the wavelength band of 700 to 800 nm, represented by PF.sub.600-700 /PF.sub.700-800. When this value is large, the internode elongation of the plant is suppressed to cause a dwarf tendency of the plant, whereas, when this ratio is large, the elongation is promoted. Actually, however, the conventional fluorescent lamps for plant growth substantially lack the light in the wavelength band of 700 to 800 nm, as shown in FIG. 2. In addition, since the fluorescent lamp for plant growth is designed to have such characteristics that the light is abundant in the wavelength band between 600 and 700 nm (see Table 1) so as to approximate the bio-synthetic curve of chlorophyll, the aforesaid ratio PF.sub.600-700 /PF.sub.700-800 of such lamp is 16.9, which is much greater than that, 1.1, of the natural light (standard daylight D.sub.65).
TABLE 1 ______________________________________ PPF per lamp input Light source (relative value) PF.sub.600-700 /PF.sub.700-800 ______________________________________ Standard daylight 556 1.1 (D.sub.65) Incandescent lamp 100 0.7 Fluorescent lamp for 243 16.9 plant growth (PG type) Three band 422 10.8 fluorescent lamp ______________________________________
Therefore, the plants grown under the light of conventional fluorescent lamps for plant growth suffer from suppression in the internode elongation and growth of the veins, thus causing dwarfing of the plants. Furthermore, as shown in FIG. 3, other ordinary illuminating fluorescent lamps, e.g., three band fluorescent lamp, substantially lack light in the wavelength band of 700 to 800 nm, so that the aforesaid ratio PF.sub.600-700 /PF.sub.700-800 is as large as 10.8, thus enhancing the dwarf tendency of the plant. It is also to be pointed out that, as shown in FIG. 2, the conventional fluorescent lamp for plant growth has a smaller amount of photosynthetic photon flux than three band fluorescent lamps in the wavelength band of 400 to 700 nm which is effective in photosynthesis.
An object of the present invention is to provide a fluorescent lamp for plant growth which is effective in growing plants similar to those grown under natural daylight in terms of morphology, i.e., height, leaf shape and size and so forth, and which effectively promotes photosynthesis.
To this end, according to the present invention, there is provided a fluorescent lamp, comprising: a glass tube; and a fluorescent layer composed of three fluorescent components having light emission peaks in the bands of 440 to 460 nm, 540 to 560 nm and 600 to 620 nm, respectively, and a far-red emission fluorescent component having peak of light emission in the wavelength band of 700 to 800 nm, the fluorescent layer having the PF ratio ranging from 0.8 to 1.2 in terms of the ratio between the photon flux PF contained in the wavelength band of 600 to 700 nm to that in the wavelength zone of 700 to 800 nm, covering the PF ratio of natural daylight (D.sub.65) which is 1.1.
According to the invention, a high photosynthesis promotion effect can be attained by using, as the fluorescent component which emits light of wavelength band of 400 to 700 nm which is effective in photosynthesis, by virtue of the use of a three band fluorescent lamp which has three types of fluorescent components activated by rare earth metals and which exhibits the highest emission efficiency among the fluorescent lamps. Furthermore, according to the present invention, a fluorescent component is used which emits far-red rays of a wavelength band of 700 to 800 nm effective in the control of plant morphology, and the ratio of the PF between the PF contained in the wavelength band of 600 to 700 nm and that contained in the wavelength band of 700 to 800 nm is determined to range between 0.8 and 1.2. Owing to these features, the present invention makes it possible to efficiently produce plants of high quality, while attaining plant morphology similar to that obtained under natural light, in terms of the height, shape and size of leaves, and so forth.